Update and rename longestConsecutive.cpp to longest-consecutive-sequence.cpp

Author: kamyu104

C++/longest-consecutive-sequence.cpp

@@ -0,0 +1,58 @@
+// Time:  O(n)
+// Space: O(n)
+
+class Solution {
+public:
+    int longestConsecutive(vector<int>& nums) {
+        // unprocessed_entries records the existence of each entry in num.
+        unordered_set<int> unprocessed_entries;
+        for (const auto& num : nums) {
+            unprocessed_entries.emplace(num);
+        }
+
+        int max_interval_size = 0;
+        while (!unprocessed_entries.empty()) {
+            int num = *unprocessed_entries.begin();
+            unprocessed_entries.erase(num);
+
+            // Finds the lower bound of the largest range containing a.
+            int lower_bound = num - 1;
+            while (unprocessed_entries.count(lower_bound)) {
+                unprocessed_entries.erase(lower_bound);
+                --lower_bound;
+            }
+
+            // Finds the upper bound of the largest range containing a.
+            int upper_bound = num + 1;
+            while (unprocessed_entries.count(upper_bound)) {
+                unprocessed_entries.erase(upper_bound);
+                ++upper_bound;
+            }
+            max_interval_size =
+            max(max_interval_size, upper_bound - lower_bound - 1);
+        }
+        return max_interval_size;
+    }
+};
+
+// Time:  O(n)
+// Space: O(n)
+class Solution2 {
+public:
+    int longestConsecutive(vector<int> &nums) {
+        if (nums.empty()) {
+            return 0;
+        }
+        unordered_map<int, int> hash;
+        int ans{1};
+        for (const auto& i : nums) {
+            if (!hash[i]) {
+                hash[i] = 1;
+                int leftbound{hash[i - 1]}, rightbound{hash[i + 1]}; // Get neighbor info.
+                hash[i - leftbound] = hash[i + rightbound] = 1 + leftbound + rightbound; // Update left and right bound info.
+                ans = max(ans, 1 + leftbound + rightbound);
+            }
+        }
+        return ans;
+    }
+};